SharePoint Offline displays a friendly error messasge that enables farm administrators to take a web application offline and display a custom message
ASP.NET 2.0 introduced the ability to take an application offline by placing a html file named app_offline.htm at the root of the web application. When this file is present, all requests are routed to the static html page. SharePoint Offline extends this ASP.NEt
2.0 feature to SharePoint, and enables you to schedule when an application will be online/offline as well as the ability to set a custom offline message for each web application.
How it Works
SharePoint Offline deploys custom actions to the Application Management page in Central Administration as well as custom administrative pages. When an administrator schedules a site to be taken offline or online, a timer job is created which deploys or retracts
the app_offline.htm file to the root of the specified web application on each server in the farm. The feature does not allow CA to be taken offline, but can also be used on My Site and the SSP administration site.